During the drying in known freeze-drying chambers with a multiplicity of stand plates for product-filled containers or planar product layers, the containers or product layers in the edge region of the stand plates exchange energy more intensively than the containers/product layers positioned in the center of the plates, on account of radiant heat exchange and natural convection in the gap between the wall of the chamber and the stack of stand plates. This non-uniformity of the energy distribution leads to a variation of freezing and drying kinetics between the containers or product layers at the edges and those in the center.
The avoidance of the non-uniformity could be achieved by eliminating the non-uniformity of the driving potential responsible for the lack of uniformity in energy distribution. The driving potential for the drying is the temperature difference between product-filled containers or product layers and their environment, which supplies the potential required for the freeze-drying to progress. In the edge region of the stand plates, this potential is greater than in the central region of the stand plates, since there is direct heat exchange between containers at the edge and the chamber wall as a result of radiation and convection. During the freezing process according to the prior art (at standard or slightly reduced pressure), the natural convection of the gas in the clear gap between the wall and the temperature-controlled stand plates has a particularly intensive action as a heat-transfer medium for the containers which are exposed to the convective flow. These additional heat fluxes decrease towards the center of the plate and thereby cause the non-uniformity in the freezing and drying of the containers or product layers distributed over the plate.
According to the prior art, freeze-dryers are either produced completely without temperature-control equipment for the chamber walls or with heating/cooling jackets which are applied directly to the supporting structure. On account of the body contact with the heavy bearing structure of the chamber, these heating/cooling jackets have the purpose of cooling the chamber from the sterilizing temperature to the temperature which is suitable for loading. Then, the cooling liquid is generally emptied from these heating/cooling surfaces, in order to reduce the mass. The cooling of the chamber wall to a temperature which eliminates the driving potential responsible for the problem is not possible with these designs.
U.S. Pat. No. 5,398,426 describes a freeze-dryer whose chamber walls can be cooled in order to eliminate the disruptive temperature differences by establishing identical temperatures at the chamber walls and the stand plates. This design has two drawbacks:    1. The additional cooling surfaces are integrated in the mechanical bearing structure of the dryer, which has to be sufficiently reinforced for it to be able to withstand evacuation. This has the drawback that large masses have to heated/cooled when the dryer is operating. Therefore, the thermal reaction of the dryer is inevitably slow.    2. The control described in U.S. Pat. No. 5,398,426, namely establishing uniform wall and stand surface temperatures, does not lead, in particular during the first drying stage, the sublimation drying, to the desired elimination of the driving potential which is responsible for the problem and therefore also does not lead to the elimination of non-uniformities, in particular during the sublimation drying.
The invention is therefore based on the following objects:                eliminating the non-uniformity between the edge region and center region of the stand plates, which during the freezing and drying of product-filled vessels leads to uneven temperature and drying profiles of the vessels;        a reduction in the mass of the dryer which has to be heated or cooled.        